1. Field of the Invention
The present invention generally relates to a circuit for driving a flyback transformer, also referred to as an ignition coil, of a spark ignited internal combustion engine. More specifically it is related to controlling the turn-on rate of the ignition coil.
2. Description of Related Art
In a spark ignited internal combustion engine, ignition coils provide the voltage required for electrical current to jump across a spark plug gap, igniting an air-fuel mixture in the engine cylinder causing combustion. A switch, also referred to as a coil driver, is used on the primary side of the ignition coil to control the charge and discharge cycles of the ignition coil.
A typical ignition system is illustrated in FIG. 1. The system includes an ignition coil 10 having a primary side 16 and a secondary side 18. The positive terminals of the primary side 16 and secondary side 18 of the ignition coil 10 are connected to a power source 12. The negative terminal of the primary side is connected to a switching transistor 14. The switching transistor 14 is connected between the ignition-coil 10 and an electrical ground 20. The negative terminal of the secondary side 18 is connected to a spark plug 22. The spark plug 22 is connected between the ignition coil 10 and an electrical ground 20.
FIG. 2 illustrates the voltage and current profiles at various points within the system. The profile of the control signal provided to the switching transistor 14 is identified by reference numeral 24. The current flowing through the primary side 16 of the ignition coil 10 is denoted by reference numeral 26. In addition, a profile of the primary coil voltage signal, as seen on the collector of transistor 14, is denoted by reference numeral 28. In a typical charge and discharge cycle the switching transistor 14 is turned on, charging the ignition coil 10 for a specified dwell period or to a specified charge current; and then the switching transistor 14 is turned off, allowing the secondary side 18 of the ignition coil 10 to discharge stored energy across the spark plug gap.
One problem is that the sharp turn-on during the charging cycle causes an oscillation on the secondary side 18 of the ignition coil 10. FIG. 3 illustrates the dwell command signal 24, the dwell current 26, the primary side (low-side) voltage 28, and the undesirable secondary voltage oscillation 30. The switching transistor 14 starts out in the off-state with the negative terminal of the primary side 16 equal to the battery voltage. After the switching transistor 14 is turned on, the transistor quickly transits through its linear range into the saturated on-state with a very large rate of voltage change across the primary side 16 of the ignition coil 10. The resulting secondary voltage 30 during turn-on is a large oscillation magnitude that decays over time. If the oscillation magnitude exceeds a tolerable level, an unintended spark event can occur across the spark plug gap, resulting in premature combustion. One way to control the magnitude of the secondary oscillations is adding constraints in design of the ignition coil 10 resulting in poor coil performance.
In view of the above, it is apparent that there exists a need for an ignition coil driving circuit with an improved ignition control.